This invention relates to a load control device in which, in response to operation signals from an operation input section having a plurality of input switches, signals are transmitted and received to control a load.
For instance in a vehicle such as an automobile, a number of signal lines or control lines are provided to drive a number of loads in response to operating signals from a number of operating switches, respectively. On the other hand, recently a method has been proposed in the art in which the number of signal lines or control lines are reduced, and operating signals are transmitted and received in a multiplex mode.
As is well known, a light switch is provided on the dashboard of an automobile. The light switch includes a plurality of positions for turning on and off the headlamps and the running lights, and an auto-mode setting switch. The auto-mode setting switch is operated for providing a mode of automatically controlling the lighting of lamps. When the auto-mode setting switch is turned on, the brightness of outside is detected with an illuminance sensor to determine whether it is "day-time", or "twilight (or down) time", or "night-time", and the lamps are controlled according to the results of determination; that is, when it is "twilight time", the running lights are lighted on, and when it is "night-time", the headlamps are turned on.
In this case, one signal line is used to control the operations of the lamps in cooperation with the switches, and signal conversion circuits are provided on the signal transmitting side and the signal receiving side, respectively. Serial signals are applied to the signal line, to allow the signal receiving side to determine which of the switches has been operated, thereby to turn on the headlamps or running lights; i.e., loads.
As is apparent from the above description, it is unnecessary to provide a plurality of signals lines for the switches. Accordingly, the labor and .time for wiring is reduced as much, and so is the manufacturing cost.
However, the above-described method is disadvantageous in the following points: For instance when the signal line is broken, or the signal transmitting side or the signal receiving side becomes out of order for some reason, it is impossible to turn on the lamps. In this case, it is impossible to turn on not only the running lights but also the headlamps. This is a serious trouble when the automobile is traveling on a dark road.
If the automobile is so designed that even when the above-described troubles occur, at least the headlamps can be turned on, then the automobile may be run during night, although it should be repaired later.
For this purpose, for instance the following means may be employed: An emergency by-pass line is provided between the headlamp lighting switch and the load control circuit so that, even in the above-described case, the lighting signal can be transmitted through the by-pass line.
However, this means is still disadvantageous in that it impairs the merit that the serial signal line is employed for multiplex signal transmission thereby to reduce the number of signal lines.